This invention relates to an apparatus and method for testing electro-optical systems under conditions which closely simulate that of real use.
In testing electro-optical systems it is customary to determine the response to a bar pattern. As discussed in "Perception of Displayed Information" (Biberman, Plenum Press, New York, 1973) operator response to such bar patterns is closely related to operator response when tracking real targets. Reference may be made to chapter 5 with particular reference to pages 224 and 225 where the correlation between "real world" objects and "equivalent bar pattern" objects is discussed.
In the testing of thermal imaging systems it has been the practice to place a bar pattern array in front of a variable temperature background in the field of the view of the system under test. The temperature difference between the bar pattern and the background is reduced until the observer can just no longer distinguish the bar pattern from the background. A curve of minimum resolvable temperature difference (MRTD) versus spatial frequency can thus be generated with observed MRTD points for different bar pattern sizes corresponding to different spatial frequencies. This method is slow and difficult to implement mainly due to the difficulties in providing suitable bar pattern sources.
An improved method, called the injection technique overcomes some of these difficulties. In this method, an electronic 4-bar pattern signal is injected at the input of the video amplifier of the system under test. The injected bar pattern signal is a sine wave gated by the synchronization signals of the imaging system. The minimum resolvable voltage (MRV) is first found and the MRTD is derived according to the following equation: ##EQU1## where MTF is the modulation transfer function of the system up to the injecton point, and .DELTA.V/.DELTA.T is the low frequency responsivity in Volt/.degree. C. The .pi./4 factor is the ratio between the sine wave signal amplitude and that of the equivalent square wave signal.
This method has two drawbacks. It is not valid at low frequencies where the higher harmonics of the square wave are not taken into account. Further, it completely neglects phase effects as well as the fact that the 4-bar pattern is a truncated signal in both dimensions. To allow an exact replacement of the earlier method by the improved method, the injected 4-bar pattern should be degraded according to the optical transfer function (OTF), which includes phase effects, in both dimensions, prior to injection.